An ongoing desire in the electronics industry is increased functionality in an ever-decreasing package size. This desire, referred to generally as miniaturization, has two primary points of focus with one being increasing the overall density of electronic components and the other being the size of the electronic components themselves. This particular invention is focused primarily on miniaturization of electronic components, specifically solid electrolytic electronic capacitors.
The contemporary capacitor market demands miniaturizing the overall capacitor case size while increasing the capacitance of the product and still maintaining the good electrical and mechanical performance parameters. To meet the demand, at the anode level, one of the approaches is to increase the charge of the powders used to form the anode while decreasing the diameter or thickness (for a flat wire) of the wire extending into the powder. While easily stated, the use of very small diameter or thickness anode wires has proven to be extremely difficult due to problems which were not previously recognized.
In practice, an anode wire is inserted into an anode powder and the anode powder is pressed into a monolith to form an anode precursor having the anode wire embedded therein and extending therefrom. The anode precursor is then sintered to form the anode. With very small anode wires the wire becomes very brittle after sintering and subsequent exposure to moisture or aqueous solutions, especially at the point of egress from the anode, leading to a very high loss rate due to anode wire breakage. The other factor that increases the wire brittleness is specific charge of the powder, CV/g. For the same diameter wire is more brittle in anodes sintered with higher CV/g powder. While not limited to theory, it is hypothesized that the wire is subjected to hydrogen evolving from the reaction of water (humidity) with tantalum in an incompletely passivated pellet. The hydrogen, as a byproduct of this reaction, diffuses into the tantalum wire making it brittle. The amount of hydrogen and, thereby, brittleness of the wire increases with the bulk concentration of oxygen in the wire [see Y. Freeman. Tantalum Capacitors: Science, Technology, and Applications, Springer, 2017]. With the increasing powder CV/g, more oxygen dissolves in the tantalum wire from the tantalum powder during the anode sintering, and thinner wire makes the bulk oxygen concentration higher.
The present invention provides a method of pretreating the anode wire, an improved anode wire and an improved capacitor wherein the anode wire is sufficiently ductile, even though very thin, thereby allowing for less anode volume being occupied by the anode wire thereby providing a higher capacitance as a function of anode volume.